Central venous catheters are essential for the care of patients requiring chronic venous access, yet clots often develop at the catheter tip and block blood withdrawal. Although heparin flushes are routinely used to prevent this, their is little evidence that they are any more effective than saline. This study compares the effectiveness of standard heparin with flushes of a new anticoagulant, lepirudin, given at least daily for the first 3 - 4 weeks after the catheter has been inserted. The measured parameter is the frequency with which the catheters develop withdrawal occlusion. Fifty-one patients have been accrued on this study, which is about to be terminated because our essential research nurse has left our department and is not being replaced. The last patient accrued has just completed her period of observation. Once her chart is reviewed for incidences of catheter withdrawal occlusion, the study will be unblinded to determine whether it had sufficient power to demonstrate significant differences between the effect